Okay, so immediately, I think you're gonna run into problems people have with this. It's kind of a pet peeve with some users, but if you can give enough of a reason for using it, you might be okay.

In reference to the list of suitable materials for use, you can use an asterisk (*) to create bullet points.

…oaken plinth, standing 4 feet high and 1 foot wide…

Please use Metric instead of Imperial units.

burning 1,200 1,600 edit: 2,000 degrees Celsius

I think, if it's really that important, should be written as "1,2001,600 2,000."

it will not burn plantlife of any kind (its own plinth being the only exception)

I think this should be changed, as it doesn't seem to actually burn the plinth, it only seems to exist on it.

memetic hazard

I don't think hazard is the right word; maybe something like "ability" or "property."

The last Addenda read as if they're messages or something. I think they should be reformatted accordingly.

While the story progression is decent, I think it could be improved. Right now, you have the "unchecked danger" cliffhanger where the flame could expand out of control, but that's seems to be about it. Raising the stakes by having it increase in temperature doesn't offer a whole lot to the reader, storywise. I don't see why the Foundation wouldn't just lock it in a heatproof chamber forever, except for the fact that it may affect the integrity of the original SCP itself.

You develop an interesting set-up with the flame acting as a god in the past, and it being able to communicate, but nothing comes to fruit of that. I think you should expand there.